Apparatus of plural charged-particle beams

ABSTRACT

One modified source-conversion unit and one method to reduce the Coulomb Effect in a multi-beam apparatus are proposed. In the modified source-conversion unit, the aberration-compensation function is carried out after the image-forming function has changed each beamlet to be on-axis locally, and therefore avoids undesired aberrations due to the beamlet tilting/shifting. A Coulomb-effect-reduction means with plural Coulomb-effect-reduction openings is placed close to the single electron source of the apparatus and therefore the electrons not in use can be cut off as early as possible.

CLAIM OF PRIORITY

This application is a continuation of application No. 16/799,773entitled “Apparatus of Plural Charged-Particle Beams,” filed Feb. 24,2020, which is a continuation of application No. 16/174,146 entitled“Apparatus of Plural Charged-Particle Beams”, filed Oct. 29, 2018, nowissued as U.S. Pat. No. 10,573,487, which is a continuation applicationof Application No. 15/403,685 entitled “Apparatus of PluralCharged-Particle Beams”, filed Jan. 11, 2017, now issued as U.S. Pat.No. 10,115,559, which is a continuation application of Application No.15/150,858, filed May 10, 2016, now issued as U.S. Pat. No. 9,607,805,which claims the benefit of priority of U.S. provisional application No.62/160,031 entitled to Xuedong Liu et al. filed on May 12, 2015 andentitled “Apparatus of Plural Charged-Particle Beams”. The disclosuresof the above-referenced applications are incorporated herein byreference in their entireties.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. application Ser. No. 15/065,342entitled to Weiming Ren et al. filed on Mar. 9, 2016 and entitled“Apparatus of Plural Charged-Particle Beams”, the entire disclosures ofwhich are incorporated herein by reference.

This application is related to U.S. application Ser. No. 15/078,369entitled to Weiming Ren et al. filed on Mar. 23, 2016 and entitled“Apparatus of Plural Charged-Particle Beams”, the entire disclosures ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a charged-particle apparatus with aplurality of charged-particle beams. More particularly, it relates to anapparatus which employs plural charged-particle beams to simultaneouslyacquire images of plural scanned regions of an observed area on a samplesurface. Hence, the apparatus can be used to inspect defects and/orparticles on wafers/masks with high detection efficiency and highthroughput in semiconductor manufacturing industry.

2. Description of the Prior Art

For manufacturing semiconductor IC chips, pattern defects and/oruninvited particles (residuals) inevitably appear on surfaces ofwafers/masks during fabrication processes, which reduce the yield to agreat degree. To meet the more and more advanced requirements onperformance of IC chips, the patterns with smaller and smaller criticalfeature dimensions have been adopted. Accordingly, the conventionalyield management tools with optical beam gradually become incompetentdue to diffraction effect, and yield management tools with electron beamare more and more employed. Compared to a photon beam, an electron beamhas a shorter wavelength and thereby possibly offering superior spatialresolution. Currently, the yield management tools with electron beamemploy the principle of scanning electron microscope (SEM) with a singleelectron beam, which therefore can provide higher resolution but can notprovide throughputs competent for mass production. Although the higherand higher beam currents can be used to increase the throughputs, thesuperior spatial resolutions will be fundamentally deteriorated byCoulomb Effect.

For mitigating the limitation on throughput, instead of using a singleelectron beam with a large current, a promising solution is to use aplurality of electron beams each with a small current. The plurality ofelectron beams forms a plurality of probe spots on one being-inspectedor observed surface of a sample. For the sample surface, the pluralityof probe spots can respectively and simultaneously scan a plurality ofsmall scanned regions within a large observed area on the samplesurface. The electrons of each probe spot generate secondary electronsfrom the sample surface where they land on. The secondary electronscomprise slow secondary electrons (energies 50 eV) and backscatteredelectrons (energies close to landing energies of the electrons). Thesecondary electrons from the plurality of small scanned regions can berespectively and simultaneously collected by a plurality of electrondetectors. Consequently, the image of the large observed area includingall of the small scanned regions can be obtained much faster thanscanning the large observed area with a single beam

The plurality of electron beams can be either from a plurality ofelectron sources respectively, or from a single electron source. For theformer, the plurality of electron beams is usually focused onto andscans the plurality of small scanned regions by a plurality of columnsrespectively, and the secondary electrons from each scanned region aredetected by one electron detector inside the corresponding column. Theapparatus therefore is generally called as a multi-column apparatus. Theplural columns can be either independent or share a multi-axis magneticor electromagnetic-compound objective lens (such as U.S. Pat. No.8,294,095). On the sample surface, the beam interval between twoadjacent beams is usually as large as 30˜50 mm.

For the latter, a source-conversion unit is used to generate a pluralityof parallel real or virtual images of the single electron source. Eachimage is formed by one part or beamlet of the primary electron beamgenerated by the single electron source, and therefore can be taken asone sub-source emitting the one beamlet. In this way, the singleelectron source is virtually changed into a plurality of sub-sourcesforming a real or virtual multi-source array. Within thesource-conversion unit, the beamlet intervals are at micro meter levelso as to make more beamlets available, and hence the source-conversionunit can be made by semiconductor manufacturing process or MEMS (MicroElectro-Mechanical Systems) process. Naturally, one primary projectionimaging system and one deflection scanning unit within one single columnare used to project the plurality of parallel images onto and scan theplurality of small scanned regions respectively, and one secondaryprojection imaging system focuses the plurality of secondary electronbeams therefrom to be respectively detected by a plurality of detectionelements of one electron detection device inside the single column. Theplurality of detection elements can be a plurality of electron detectorsplaced side by side or a plurality of pixels of one electron detector.The apparatus with a plurality of beamlets therefore is generally calledas a multi-beam apparatus and the conventional apparatus with a singleelectron beam is called as a single-beam apparatus.

Conventionally, the source-conversion unit comprises one image-formingmeans, and one beamlet-forming means or one beamlet-limit means. Theimage-forming means basically comprises a plurality of image-formingelements, and each image-forming element can be a round lens or adeflector. The beamlet-forming means and the beamlet-limit means arerespectively above and below the image-forming means and have aplurality of beamlet-limit openings. In one source-conversion unit withone beamlet-forming means, at first the plurality of beamlet-limitopenings divides the primary electron beam into a plurality of beamlets,and then the plurality of image-forming elements (round lenses ordeflectors) focuses or deflects the plurality of beamlets to form theplurality of parallel real or virtual images. U.S. Pat. Nos. 7,244,949and 6,943,349 respectively propose an multi-beam apparatus with onesource-conversion unit of this type. In one source-conversion unit withone beamlet-limit means, at first the plurality of image-formingelements (deflectors) deflects a plurality of beamlets of the primaryelectron beam to form the plurality of parallel virtual images, and thenthe plurality of beam-limit openings cuts off peripheral electrons ofthe plurality of beamlets respectively. The first cross referenceproposes a multi-beam apparatus 100A with one source-conversion unit ofthis type, as shown in FIG. 1 .

In FIG. 1 , for sake of simplification, the primary projection imagingsystem 130 is not shown in detail and the secondary projection imagingsystem and the electron detection device are not shown. The singleelectron source 101 on the primary optical axis 100_1 generates theprimary electron beam 102 seemingly coming from the crossover 101 s. Thecondenser lens 110 focuses the primary electron beam 102 and therebyforming an on-axis virtual image 101 sv of the crossover 101 s. Theperipheral electrons of the primary electron beam 102 are cut off by themain opening of the main aperture plate 171. The source-conversion unit120 comprises the image-forming means 122 with three image-formingelements 122_1, 122_2 and 122_3, and a beamlet-limit means 121 withthree beam-limit openings 121_1, 121_2 and 121_3. Each image-formingelement functions as one micro-deflector. The beam-limit opening 121_1is aligned with the primary optical axis 100_1, and therefore theimage-forming element 122_1 is not necessary to comprise onemicro-deflector. The image-forming elements 122_2 and 122_3 respectivelydeflect beamlets 102_2 and 102_3 of the primary electron beam 102, andthereby forming two off-axis virtual images 102_2 v and 102_3 v of thecrossover 101 s. The deflected beamlets 102_2 and 102_3 areperpendicularly incident onto the beamlet-limit means 121. Thebeam-limit openings 121_1, 121_2 and 121_3 respectively cut off theperipheral electrons of the center beamlet 102_1 and the deflectedbeamlets 102_2 and 102_3, and thereby limiting the currents thereof. Thefocusing power of the condenser lens 110 varies the current density ofthe primary electron beam 102, and therefore is able to change thecurrents of the beamlets 102_1˜102_3. Consequently, three parallelvirtual images 101 sv, 102_2 v and 102_3 v form one virtual multi-sourcearray 101 v with variable currents. The primary projection imagingsystem 130 then images the virtual multi-source array 101 v onto thebeing-observed surface 7 of the sample 8 and therefore form three probespots 102_1 s, 102_2 s and 102_3 s thereon. Each of the image-formingelements 122_1˜122_3 can have a dipole configuration (with twoelectrodes) which can generate one deflection field in its requireddeflection direction, or a quadrupole or 4-pole configuration (with fourelectrodes) which can generate one deflection field in any direction.

To compensate the inherent off-axis aberrations and the manufacturingderivative aberrations of the probe spots 102_1 s, 102_2 s and 102_3 s,each of the image-forming elements 122_1, 122_2 and 122_3 can furtherfunction as one micro-compensator to compensate the field curvatureaberration and the astigmatism aberration. Accordingly, eachimage-forming element can have a 4-pole lens (with four electrodes whoseinner surfaces form a cylindrical surface) which can generate onedeflection field in any direction, one quadrupole field in one specificdirection and one round-lens field, or octupole or 8-pole lens (witheight electrodes whose inner surfaces form a cylindrical surface) whichcan generate one deflection field and one quadrupole field in anydirections. The 4-pole lens needs to be oriented to make the specificdirection of the quadrupole field match the direction of the astigmatismaberration. If a lot of beamlets is used, it may be difficult tomanufacture a large number of 8-pole lenses or 4-pole lenses each with aspecific orientation. In this case, for each beamlet, a pair of 4-polelenses (such as 122_2 dc-1 and 122_2 dc-2) can be used in the way shownin FIGS. 2A˜2C. The upper and lower 4-pole lenses of one pair of 4-polelenses are respectively placed in the upper and lower layers 122-1 and122-2, aligned with each other and have a 45° difference in azimuth ororientation. For each image-forming element, the deflection field in anydesired deflection direction and the round-lens field can be generatedby either or both of the upper and lower 4-pole lenses, and thequadrupole field in any desired compensation direction can be generatedby both of the upper and lower 4-pole lenses.

In FIG. 1 and FIG. 2A, each image-forming element performs theimage-forming function and aberration-compensation functionsimultaneously. The performance of the aberration-compensation functionwith respect to a tilting beamlet will be inferior to a normal beamlet,and this will finally limit the available image resolutions of theapparatus. Accordingly, it is necessary to provide an apparatus ofplural charged-particle beams, which can avoid the foregoing performancedeterioration.

SUMMARY OF THE INVENTION

The object of this invention is to modify one source-conversion unit ofone multi-beam apparatus in CROSS REFERENCE to avoid generatingundesired aberrations during performing the aberration-compensationfunction. In the modified source-conversion unit, the image-formingfunction and the aberration-compensation function are done separately.The aberration-compensation function is carried out after theimage-forming function has changed each beamlet to be locally on-axiswith respect to one corresponding micro-compensator, and thereforeavoids additional aberrations due to beamlet tilting/shifting. Theinvention also proposes a method to reduce the impact of Coulomb Effectas much as possible in one multi-beam apparatus. ACoulomb-effect-reduction means with plural Coulomb-effect-reductionopenings is placed close to the single electron source of the apparatusand therefore the electrons not in use can be cut off as early aspossible. Consequently, the image resolution of the apparatus can beimproved.

Accordingly, the invention therefore provides a source-conversion unit,which comprises means for forming a plurality of virtual images of asingle charged-particle source which generates a charged-particle beam,by using a plurality of micro-deflectors and a plurality ofmicro-compensators in sequence. The plurality of micro-deflectorsconverts the single charged-particle beam into a plurality of parallelbeamlets and forms the plurality of virtual images respectively, and theplurality of micro-compensators compensates aberrations of the pluralityof virtual images. The source-conversion unit also comprises means forlimiting a current of each of the plurality of beamlets.

The present invention provides a source-conversion unit of an electronsource, which comprises an image-forming means, and a beamlet-limitmeans with a plurality of beamlet-limit openings. The image-formingmeans comprises a micro-deflector array with a plurality ofmicro-deflectors and a micro-compensator array with a plurality ofmicro-compensators, and each micro-deflector is aligned with onemicro-compensator and one beamlet-limit opening. That eachmicro-deflector can deflect one beamlet of an electron beam generated bythe electron source to forms one virtual image thereof and enter thatone micro-compensator along an optical axis thereof. That onemicro-compensator can influence that one beamlet to add certain amountsof astigmatism aberration and/or field curvature aberration to thevirtual image. That one beamlet-limit opening cuts off peripheralelectrons of that one beamlet and thereby limiting a current thereof.The beamlet-limit means is preferred below the micro-deflector array.

That one micro-compensator comprises a plurality of combined submicro-compensators. The micro-compensator array comprises a firstmicro-compensator layer with a plurality of first sub micro-compensatorsand a second micro-compensator layer with a plurality of second submicro-compensators, one first sub micro-compensator and one second submicro-compensator aligned with each other are two of the plurality ofcombined sub micro-compensators.

For that one micro-compensator, the first sub micro-compensator and thesecond sub micro-compensator can be respectively a 4-pole lens and havea 45° difference in orientation. The micro-compensator array may furthercomprises a third micro-compensator layer with a plurality of third submicro-compensators, and one third sub micro-compensator aligned with thesecond sub micro-compensator is one of the plurality of combined submicro-compensators. In this case, for that one micro-compensator, thefirst sub micro-compensator and the second sub micro-compensator arerespectively a 4-pole lens and have a 45° difference in orientation, andthe third sub micro-compensator is a round-lens.

The source-conversion unit may further comprise a first-upperelectric-conduction plate with a plurality of first-upper through-holesand a first-lower electric-conduction plate with a plurality offirst-lower through-holes, which are respectively above and belowelectrodes of the plurality of micro-deflectors to avoid radiationdamage due to the electron beam and keep electric fields thereoftherebetween.

The source-conversion unit may further comprise a second-upperelectric-conduction plate with a plurality of second-upper through-holesand a second-lower electric-conduction plate with a plurality ofsecond-lower through-holes, which are respectively above and belowelectrodes of the plurality of micro-compensators to avoid radiationdamage due to corresponding beamlets and keep electric fields thereoftherebetween.

The present invention also provides a multi-beam apparatus, whichcomprises an electron source, a condenser lens below the electronsource, a source-conversion unit below the condenser lens, a primaryprojection imaging system below the source-conversion unit andcomprising an objective lens, a deflection scanning unit inside theprimary projection imaging system, a sample stage below the primaryprojection imaging system, a beam separator above the objective lens, asecondary projection imaging system above the beam separator, and anelectron detection device with a plurality of detection elements. Thesource-conversion unit comprises an image-forming means and abeamlet-limit means with a plurality of beam-limit openings. Theimage-forming means comprises a micro-deflector array with a pluralityof micro-deflectors and a micro-compensator array with a plurality ofmicro-compensators. The beamlet-limit means is below saidmicro-deflector array. An optical axis of each micro-compensator isparallel to a primary optical axis of the apparatus and aligned with oneof the plurality of micro-deflectors and one of the plurality ofbeamlet-limit openings. The electron source, the condenser lens, thesource-conversion unit, the primary projection imaging system, thedeflection scanning unit and the beam separator are aligned with theprimary optical axis. The secondary projection imaging system and theelectron detection device are aligned with a secondary optical axis ofthe apparatus, and the secondary optical axis is not parallel to theprimary optical axis. The sample stage can sustain a sample with abeing-observed surface facing to the objective lens. The electron sourcecan generate a primary electron beam along the primary optical axis. Theplurality of micro-deflectors respectively deflects a plurality ofbeamlets of the primary electron beam to be incident onto the pluralityof micro-compensators along optical axes thereof and therefore form aplurality of parallel virtual images of the electron source. Theplurality of micro-compensators respectively influences the plurality ofbeamlets with certain amounts of astigmatism aberrations and/or fieldcurvature aberrations. The plurality of beam-limit openings respectivelycuts off peripheral electrons of the plurality of beamlets and thereforelimits currents thereof, and the currents can be varied together byadjusting focusing power of the condenser lens. The primary projectionimaging system focuses the plurality of beamlets, images the pluralityof parallel virtual images onto the surface and forms a plurality ofprobe spots thereon. The certain amounts of astigmatism aberrationsand/or field curvature aberrations compensate astigmatism aberrationsand/or field curvature aberrations generated by the condenser lensand/or the primary projection imaging system so as to reduce sizes ofthe plurality of probe spots. The deflection scanning unit deflects theplurality of beamlets to scan the plurality of probe spots respectivelyover a plurality of scanned regions within an observed area on thesurface. A plurality of secondary electron beams is generated by theplurality of probe spots respectively from the plurality of scannedregions and in passing focused by the objective lens. The beam separatorthen deflects the plurality of secondary electron beams to the secondaryprojection imaging system. The secondary projection imaging systemfocuses and keeps the plurality of secondary electron beams to bedetected by the plurality of detection elements respectively, and eachdetection element therefore provides an image signal of onecorresponding scanned region.

That each micro-compensator can comprise a plurality of combined submicro-compensators. Therefore the micro-compensator array can comprise afirst micro-compensator layer with a plurality of first submicro-compensators and a second micro-compensator layer with a pluralityof second sub micro-compensators, and one first sub micro-compensatorand one second sub micro-compensator aligned with each other are two ofthe plurality of combined sub micro-compensators. (claim 13) For thateach micro-compensator, the first sub micro-compensator and the secondsub micro-compensator are respectively a 4-pole lens and have a 45°difference in orientation.

The micro-compensator array may further comprise a thirdmicro-compensator layer with a plurality of third submicro-compensators, and one third sub micro-compensator aligned with thesecond sub micro-compensator is one of the plurality of combined submicro-compensators. For that each micro-compensator, the first submicro-compensator and the second sub micro-compensator can berespectively a 4-pole lens and have a 45° difference in orientation, andthe third sub micro-compensator can be a round-lens.

The image-forming means may comprise a first-upper electric-conductionplate with a plurality of first-upper through-holes and a first-lowerelectric-conduction plate with a plurality of first-lower through-holes,which are respectively above and below electrodes of the plurality ofmicro-deflectors to avoid radiation damage due to the primary electronbeam and keep electric fields thereof therebetween.

The image-forming means may comprise a second-upper electric-conductionplate with a plurality of second-upper through-holes and a second-lowerelectric-conduction plate with a plurality of second-lowerthrough-holes, which are respectively above and below electrodes of theplurality of micro-compensators to avoid radiation damage due tocorresponding beamlets and keep electric fields thereof therebetween.

The multi-beam apparatus may further comprise a Coulomb-effect-reductionmeans which is between the electron source and the condenser lens andcomprises a plurality of Coulomb-effect-reduction openings, wherein mostelectrons within the primary electron beam and not constituting theplurality of beamlets, will not pass through the plurality ofCoulomb-effect-reduction openings.

The present invention also provides a method for converting a singlecharged particle source into a plurality of virtual sub-sources, whichcomprises steps of deflecting a charged-particle beam of the singlecharged particle source into a plurality of parallel beamlets whichforms the plurality of virtual images respectively, correctingaberrations of the plurality of virtual, and cutting a current of eachof the plurality of beamlets. Each of the plurality of virtual images isone of the plurality of sub-sources

The present invention provides a method to configure a source-conversionunit of an electron source, which comprises steps of providing animage-forming means with a micro-deflector array and a micro-compensatorarray, and providing a beamlet-limit means with a plurality ofbeamlet-limit openings. The micro-deflector array comprises a pluralityof micro-deflectors, and the micro-compensator array comprises aplurality of micro-compensators. Each micro-deflector is aligned withone micro-compensator and one beamlet-limit opening. That eachmicro-deflector deflects one beamlet of an electron beam generated bythe electron source to form one virtual image thereof and is incidentonto that one micro-compensator along an optical axis thereof. That onemicro-compensator influences that one beamlet to add certain amounts ofastigmatism aberration and/or field curvature aberration to that onevirtual image. That one beamlet-limit opening cuts off peripheralelectrons of that one beamlet and thereby limiting a current thereof.

The method may further comprise a step of providing a first-upperelectric-conduction plate with a plurality of first-upper through-holesand a first-lower electric-conduction plate with a plurality offirst-lower through-holes respectively, which are respectively above andbelow electrodes of the plurality of micro-deflectors to avoidgenerating radiation damage due to the electron beam and keep electricfields thereof therebetween.

The method may further comprise a step of providing a second-upperelectric-conduction plate with a plurality of second-upper through-holesand a second-lower electric-conduction plate with a plurality ofsecond-lower through-holes, which are respectively above and belowelectrodes of the plurality of micro-compensators to avoid generatingradiation damage due to corresponding beamlets and keep electric fieldsthereof therebetween.

The present invention provides a method to obtain a plurality ofsub-sources from an electron source, which comprises steps of performingan image-forming function by a plurality of micro-deflectors, performingan aberration-compensation function by a plurality ofmicro-compensators, and performing a current-limit function by pluralityof beamlet-limit apertures. The image-forming function provides aplurality of parallel virtual images of the electron source and eachvirtual image becomes one of the plurality of sub-sources. Theaberration-compensation function adds certain amounts of specificaberrations to the plurality of sub-sources respectively, and thecurrent-limit function limits currents of the plurality of sub-sourcesrespectively. The plurality of micro-deflectors respectively deflect aplurality of beamlets of an electron beam generated by the electronsource, and the plurality of beamlets forms the plurality of parallelvirtual images and enters the plurality of micro-compensators alongoptical axes thereof. The plurality of micro-compensators respectivelyinfluences the plurality of beamlets with the certain amounts ofspecific aberrations. The plurality of beamlet-limit aperturesrespectively cuts off peripheral electrons of the plurality of beamletsto limit currents thereof.

Other advantages of the present invention will become apparent from thefollowing description taken in conjunction with the accompanyingdrawings wherein are set forth, by way of illustration and example,certain embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be readily understood by the followingdetailed description in conjunction with the accompanying drawings,wherein like reference numerals designate like structural elements, andin which:

FIG. 1 is a schematic illustration of one configuration of aconventional multi-beam apparatus.

FIGS. 2A˜2C are schematic illustrations of one conventionalconfiguration of one image-forming means in a conventionalsource-conversion unit.

FIG. 3 is a schematic illustration of a configuration of a newmulti-beam apparatus in accordance with one embodiment of the presentinvention.

FIG. 4 , FIG. 5A and FIG. 5B are respectively a schematic illustrationof one configuration of the micro-compensation array in FIG. 3 .

FIGS. 6A, 6B and 6C are schematic illustrations of one part of theconfiguration of the micro-compensation array in FIG. 5A or FIG. 5B.

FIG. 7 is a schematic illustration of a configuration of a newmulti-beam apparatus in accordance with another embodiment of thepresent invention.

FIG. 8 is a schematic illustration of a configuration of a newmulti-beam apparatus in accordance with another embodiment of thepresent invention.

FIG. 9 is a schematic illustration of a configuration of a newmulti-beam apparatus in accordance with another embodiment of thepresent invention.

FIGS. 10A and 10B are respectively a schematic illustration of oneembodiment of the source-conversion unit in FIG. 3 and FIG. 7 .

FIGS. 11A and 11B are respectively a schematic illustration of oneembodiment of the source-conversion unit in FIG. 8 .

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Various example embodiments of the present invention will now bedescribed more fully with reference to the accompanying drawings inwhich some example embodiments of the invention are shown. Withoutlimiting the scope of the protection of the present invention, all thedescription and drawings of the embodiments will exemplarily be referredto an electron beam. However, the embodiments are not be used to limitthe present invention to specific charged particles.

In the drawings, relative dimensions of each component and among everycomponent may be exaggerated for clarity. Within the followingdescription of the drawings the same or like reference numbers refer tothe same or like components or entities, and only the differences withrespect to the individual embodiments are described. For sake ofclarity, only three beamlets are available in the drawings, but thenumber of beamlets can be anyone.

Accordingly, while example embodiments of the invention are capable ofvarious modifications and alternative forms, embodiments thereof areshown by way of example in the drawings and will herein be described indetail. It should be understood, however, that there is no intent tolimit example embodiments of the invention to the particular formsdisclosed, but on the contrary, example embodiments of the invention areto cover all modifications, equivalents, and alternatives falling withinthe scope of the invention.

In this invention, “axial” means “in the optical axis direction of alens (round or multi-pole), an imaging system or an apparatus”, “radial”means “in a direction perpendicular to the optical axis”, “on-axial”means “on or aligned with the optical axis” and “off-axis” means “not onor not aligned with the optical axis”.

In this invention, “an imaging system is aligned with an optical axis”means “all the electron optics elements (such round lens and multi-polelens) are aligned with the optical axis”.

In this invention, X, Y and Z axe form Cartesian coordinate. The opticalaxis of the primary projection imaging system is on the Z-axis, and theprimary electron beam travels along the Z-axis.

In this invention, all terms relate to through-holes, openings andorifices mean openings or holes penetrated through one plate.

In this invention, “primary electrons” means “electrons emitted from anelectron source and incident onto a being-observed or inspected surfaceof a sample, and “secondary electrons” means “electrons generated fromthe surface by the “primary electrons”.

Basic configurations of two types of multi-beam apparatuses forobserving or inspecting a surface of a sample are disclosed in CROSSREFERENCE. In one first-type apparatus, the surface is perpendicular tothe optical axis thereof as shown in FIG. 1 , and in one second-typeapparatus the surface tilts with respect to the optical axis thereof.The methods and the embodiments proposed in the present invention can beused in both one first-type apparatus and one second-type apparatus soas to improve the performances thereof. In the next descriptions, onlythe first-type apparatus is taken as an example. For sake ofsimplification, the primary projection imaging system, the secondaryprojection imaging system and the electron detection device are notshown and not mentioned in the illustrations and the description of theembodiments respectively.

FIG. 3 shows one embodiment of the modified source-conversion unit 120in one first-type apparatus 200A. The single electron source 101, thecommon condenser lens 110, the main aperture plate 171, thesource-conversion unit 120, and the primary projection imaging system130 are placed along and aligned with the primary optical axis 200_1.The source-conversion unit 120 comprises one image-forming means 122 andone beamlet-limit means 121 with three beamlet-limit apertures (oropenings) 121_1, 121_2 and 121_3. The image-forming means 122 comprisesone micro-deflector array 122_D with three micro-deflectors 122_1D,122_2D and 122_3D, and one micro-compensator array 122_C with threemicro-compensators 122_1C, 122_2C and 122_3C. The beamlet-limit means121 is a beamlet-limit-aperture array. The optical axes of the threemicro-deflectors and the three micro-compensators are parallel to theprimary optical axis 200_1, and each of the three micro-deflectors isaligned with one of the three micro-compensators and one of the threebeamlet-limit openings. For example, the micro-deflector 122_2D isaligned with the micron-compensator 122_2C and the beamlet-limit opening121_2. Here the micro-deflector 122_1D, the micro-compensator 122_1C andthe beamlet-limit opening 121_1 are shown on the primary optical axis200_1, but they can be off the primary optical axis 200_1.

The single electron source 101 generates a primary electron beam 102with high energy (such as 8˜20 keV), a high angular intensity (such as0.5˜5 mA/sr) and a crossover (virtual or real) 101 s shown by theon-axis oval mark here. Therefore it is convenient to think that theprimary electron beam 102 is emitted from the crossover 101 s, and thesingle electron source 101 is simplified to be the crossover 101 s.

The primary electron beam 102 passes through the condenser lens 110without focusing influence and its peripheral electrons are cut off bythe main opening of the main aperture plate 171. The micro-deflectors122_2D and 122_3D respectively deflect beamlets 102_2 and 102_3 of theprimary electron beam 102 to be parallel or substantially parallel tothe primary optical axis 200_1. The deflected beamlets 102_2 and 102_3respectively form the off-axis virtual images 102_2 v and 102_3 v of thecrossover 101 s of the single electron source 101. Then the threebeamlets 102_1, 102_2 and 102_3 pass through the correspondingmicro-compensators 122_1C, 122_2C and 122_3C along the optical axesthereof. Each micro-compensator generates a round-lens field and aquadrupole field, and thereby accordingly adding specific values offocusing or field curvature aberration and astigmatism aberration to thecorresponding beamlet. In this way, each micro-compensator avoidsgenerating additional aberrations due to beamlet tilting and/orshifting.

Next the beamlet-limit openings 121_1, 121_2 and 121_3 cut off theperipheral electrons of the beamlets 102_1, 102_2 and 102_3respectively, and thereby limiting the currents thereof. The primaryprojection imaging system 130 focuses the beamlets 102_1, 102_2 and102_3 and accordingly images the crossover 101 s and its two paralleloff-axis virtual images 102_2 v and 102_3 v onto the being-observedsurface 7 and therefore forms three probe spots thereon. For each probespot, the specific values of the field curvature aberration andastigmatism aberration generated by the corresponding micro-compensatorcompensate the corresponding aberrations due to the primary projectionimaging system 130. The currents of the three probe spots can be variedby turning on the condenser lens 110 to focus the primary electron beam102 to a certain degree. Increasing the focusing power of the condenserlens 110 will increase the current density of the primary electron beam102 incident onto the source-conversion unit 120, and accordinglyincrease the currents of the three beamlets 102_1, 102_2 and 102_3. Inthis case, the foregoing specific values of the field curvatureaberration and astigmatism aberration will compensate the correspondingaberrations due to the condenser lens 110 and the primary projectionimaging system 130.

Same as the prior art, each of the three micro-deflectors 122_1D, 122_2Dand 122_3D can have a dipole configuration (with two electrodes) whichcan generate one deflection field in its required deflection direction,or a quadrupole or 4-pole configuration (with four electrodes) which cangenerate one deflection field in any direction.

Each of the three micro-compensators 122_1C, 122_2C and 122_3C can be a4-pole lens (with four electrodes whose inner surfaces form acylindrical surface) with a specific orientation, and therefore cangenerate one round-lens field and one quadrupole field in the directionof the compensated astigmatism aberration as shown in FIG. 4 . For thesake of clarity, three more micro-compensators are shown. Themicro-compensator 122_3C is oriented to generate a quadrupole field in Xdirection, and the micro-compensator 122_31C is oriented to generate aquadrupole field in the vector 122_31C_2 direction. Each foregoingmicro-compensator can also be an octupole or 8-pole lens (with eightelectrodes whose inner surfaces form a cylindrical surface) which cangenerate a quadrupole field in any directions and one round-lens field.In this case, all the micro-compensators can have same configurationsand be placed in same orientations.

For an apparatus using a lot of beamlets, it may be difficult tomanufacture a large number of 8-pole lenses or 4-pole lenses each with aspecific orientation. In addition, for one micro-compensator tocompensate both field curvature aberration and astigmatism aberration,the excitation voltages of the multiple electrodes may be larger thanthe safety limitations of electric breakdown. To solve the foregoingissues, each micro-compensator can be formed by two or more submicro-compensators, and accordingly is called as a combined one. In thiscase, the micro-compensator array 122_C can be formed by two or moremicro-compensator layers placed along the primary optical axis 200_1.Each micro-compensator layer has a plurality of sub micro-compensators,each sub micro-compensator in one layer is aligned with one submicro-compensator in every other layer, and all the submicro-compensators aligned with each other form one foregoing combinedmicro-compensator. FIG. 5A and FIG. 5B respectively show onemicro-compensator array 122_C with two and three micro-compensatorlayers.

In the micro-compensator array 122_C in FIG. 5A, counting from thebeamlet entering side, the first micro-compensator layer 122_C-1 hasthree first sub micro-compensators 122_1C-1, 122_2C-1 and 122_3C-1, andthe second micro-compensator layer 122_C-2 has three second submicro-compensators 122_1C-2, 122_2C-2 and 122_3C-2. One first submicro-compensator and one second micro-compensator are aligned with eachother and form a combined micro-compensator with respect to one beamlet.For example, the first sub micro-compensator 122_2C-1 and the second submicro-compensator 122_2C-2 form one combined micro-compensator 122_2C inFIG. 3 with respect to the beamlet 102_2 and are aligned with theoptical axis 122_2C_1 thereof.

For each combined micro-compensator in FIG. 5A, in one case the firstsub micro-compensator and the second sub micro-compensator canrespectively be a 4-pole lens and have a 45° difference in azimuth ororientation. For example, the first and second micro-compensator layerscan have the configurations 122_C-n shown in FIG. 6A and FIG. 6Brespectively, wherein all the sub micro-compensators in each layer aresame in orientation. In this way, the round-lens field for compensatingthe field curvature aberration can be generated by either or both of theupper and lower 4-pole lenses, and the quadrupole field for compensatingthe astigmatism aberration can be generated by both of the upper andlower 4-pole lenses. In another case, one of the first submicro-compensator and the second sub micro-compensator can be a 4-polelens in a specific orientation or an 8-pole lens in any orientation andthe other can be a round lens. For example the first and secondmicro-compensator layers can have the configurations in FIG. 4 and FIG.6C. The 4-pole lens or the 8-pole lens generates the quadrupole fieldfor compensating the astigmatism aberration, and the round lensgenerates the round-lens field for compensating the field curvatureaberration. In FIG. 6C, each round lens such as 122_2C-n is formed byone circular electrode. If the compensated field curvature aberration isvery large, the excitation voltage of the circular electrode in thecorresponding round lens may be so large that a discharge is easy tohappen. In this situation, the 4-pole lens or the 8-pole lens canfurther generate an auxiliary round-lens field, the round lens generatesa main round-lens field, and the field curvature aberration iscompensated by both auxiliary and main round-lens fields. By this way,the excitation voltage of the circular electrode can be reduced andwithin a safe range.

In comparison with FIG. 5A, the micro-compensator array 122_C in FIG. 5Bfurther comprises one third micro-compensator layer 122_C-3 with threethird sub micro-compensators 122_1C-3, 122_2C-3 and 122_3C-3. One firstsub micro-compensator, one second sub micro-compensator and one thirdsub micro-compensator are aligned with each other and form a combinedmicro-compensator with respect to one beamlet. For example, the first,second and third sub micro-compensators 122_2C-1, 122_2C-2 and 122_2C-3form one combined micro-compensator 122_2C in FIG. 3 with respect to thebeamlet 102_2 and are aligned with the optical axis 122_2C_1 thereof.

For each combined micro-compensator in FIG. 5B, in one case, one of thefirst, second and third sub micro-compensators can be a round lens, andthe others can respectively be a 4-pole lens and have a 45° differencein azimuth or orientation. As mentioned above, the field curvatureaberration can be compensated by the round lens only or the round lensand one or both of the two 4-pole lenses together, and the astigmatismaberration can be compensated by both of the two 4-pole lenses. Inanother case, one of the first, second and third sub micro-compensatorscan be a 4-pole lens in a specific orientation or an 8-pole lens, andthe others can respectively be a round lens. In this way, the fieldcurvature aberration can be compensated by the two round lenses only orthe tow round lenses and the 4-pole or 8-pole lens together, and theastigmatism aberration can be compensated by the 4-pole or 8-polelenses.

FIG. 7 shows another embodiment of the modified source-conversion unit120 in one first-type apparatus 300A, which comprises onemicro-deflector-and-compensator array 122_DC and one micro-compensatorarray 122_C. The micro-deflector-and-compensator array 122_DC deflects aplurality of beamlets to form a plurality of virtual images of thesingle electron source 101 s, and compensates parts of the fieldcurvature and/or astigmatism aberrations of the plurality of probe spotson the sample surface 7. The micro-compensator array 122_C compensatesthe left parts of the field curvature and/or astigmatism aberrations.Accordingly the micro-deflector-and-compensator array 122_DC andmicro-compensator array 122_C can employ some configurations in CROSSREFERENCE or mentioned above. For example, in one case, themicro-compensator array 122_C can comprise a plurality of round lensesas shown in FIG. 6C, and the micro-deflector-and-compensator array122_DC can comprise a plurality of 4-pole lenses each in a specificorientation, or a plurality of 8-pole lenses, or a plurality of pairs of4-pole lenses as described in CROSS REFERENCE.

FIG. 8 shows another embodiment of the modified source-conversion unit120 in one first-type apparatus 400A. Different from the modifiedsource-conversion 120 in FIG. 3 , the beamlet-limit means 121 is placedbetween the micro-deflector array 122_D and the micro-compensator array122-C. Because the peripheral electrons of each beamlet have been cutoff before entering the corresponding micro-compensator, the damagesthereof (such as charging up and contamination) due to the peripheralelectrons can be avoided. In addition, the beamlet-limit means 121 canalso be above the image-forming means 122. In this case, the CoulombEffect can be reduced earlier than before, but the scattering electronsgenerated when each beamlet passes through one image-forming element ofthe image-forming means will enlarge the corresponding probe spot and/orbecome a background noise.

To further reduce the impact of Coulomb Effect, the main aperture plate171 in one multi-beam apparatus can be placed above the condenser lens110 and preferred as close to the single electron source 101 aspossible. In this way, peripheral electrons can be cut off as earlier aspossible. To further cut the electrons not in use as much as possible,the main aperture plate 171 with one large opening can be replaced by aCoulomb-effect-reduction means 172 with plural Coulomb-effect-reductionopenings, as shown in FIG. 9 . The primary electron beam 102 is changedinto three beamlets 102_1, 102_2 and 102_3 by threeCoulomb-effect-reduction openings, and the currents of the threebeamlets 102_1, 102_2 and 102_3 are limited by the three beamlet-limitopenings of the beamlet-limit means 121.

In the foregoing embodiments of the modified source-conversion unit 120,to operate one micro-deflector, a driving-circuit needs connecting witheach electrode thereof. To prevent the driving-circuit from beingdamaged by the primary electron beam 102 or the beamlet, it is betterplacing one electric-conduction plate above the electrodes of all themicro-deflectors. In addition, the deflection of each beamlet is betterfinished within a limited range so as to ensure a normal incidence ontothe corresponding beamlet-limit opening and/or micro-compensator.Therefore it is better to use two electric-conduction plates to sandwichthe multiple electrodes of every micro-deflector.

Similarly, to operate one micro-compensator, a driving-circuit needsconnecting with each electrode thereof. To prevent the driving-circuitsfrom being damaged by the beamlet and/or the scattered electronsthereof, it is better placing one electric-conduction plate above theelectrodes of all the micro-compensators. The aberration compensation ofeach beamlet is better finished within a limited range so as to avoidthe interferences with the other electron optical elements (such as theprimary projection imaging system, other micro-compensators ormicro-deflectors). The interferences will incur additional aberrations.Therefore it is better to use two electric-conduction plates to sandwichthe multiple electrodes of every micro-compensator.

Accordingly, FIG. 10A and FIG. 10B respectively show one overallembodiment of the source-conversion unit 120 in FIG. 3 . Thebeamlet-limit means 121 is a beamlet-limit electric-conduction platewith plural beamlet-limit openings (such as 121_1˜121_3). In themicro-deflector array 122_D in FIG. 10A, the electrodes of the pluralmicro-deflectors (such as 122_1D˜122_3D) are sandwiched by onefirst-upper insulator plate 122-IL1 with plural first-upper orifices andone first-lower insulator plate 122-IL2 with plural first-lowerorifices, and one first-upper electric-conduction plate 122-CL1 withplural first-upper through-holes and one first-lower electric-conductionplate 122-CL2 with plural first-lower through-holes sandwich thefirst-upper insulator plate 122-IL1 and the first-lower insulator plate122-IL2. The plural first-upper through-holes, the plural first-upperorifices, the plural first-lower orifices and the plural first-lowerthrough-holes are aligned with the plural micro-deflectors respectively.To avoid charging-up on the inner sidewalls of the plural first-upperand first-lower orifices, the plural first-upper through-holes are equalto or smaller than the plural first-upper orifices in radial dimensionsrespectively, and the radial dimensions of the plural first-lowerorifices are larger than the inner radial dimensions of the electrodesof the plural micro-deflectors respectively.

In the micro-compensator array 122_C in FIG. 10A, the electrodes of theplural micro-compensators (such as 122_1C-122_3C) are sandwiched by onesecond-upper insulator plate 122-IL3 with plural second-upper orificesand one second-lower insulator plate 122-IL4 with plural second-lowerorifices, and one second-upper electric-conduction plate 122-CL3 withplural second-upper through-holes and one second-lowerelectric-conduction plate 122-CL4 with plural second-lower through-holessandwich the second-upper insulator plate 122-IL3 and the second-lowerinsulator plate 122-IL4. The plural second-upper through-holes, theplural second-upper orifices, the plural second-lower orifices and theplural second-lower through-holes are aligned with the pluralmicro-compensators respectively. To avoid charging-up on the innersidewalls of the plural second-upper and second-lower orifices, theplural second-upper through-holes are equal to or smaller than theplural second-upper orifices in radial dimensions respectively, and theradial dimensions of the plural second-lower orifices are larger thanthe inner radial dimensions of the electrodes of the pluralmicro-compensators respectively.

To reduce the possibility of beamlet incurring electron scattering, eachfirst-upper through-hole, each second-upper through-hole and eachbeamlet-limit opening are respectively preferred in an upside-downfunnel shape (i.e. the small end is on the beamlet-incident side).

The embodiment of the source-conversion unit 120 in FIG. 10A can becompacted for simplifications in structure and manufacturing. Forexample, the micro-deflector array 122_D, the micro-compensator 122_Cand the beamlet-limit electric-conduction plate 121 can be placed toconnect together. That is the second-upper electric-conduction plate122-CL3 touches the first-lower electric-conduction plate 122-CL2, andthe beamlet-limit electric-conduction plate 121 is attached to thesecond-lower electric-conduction plate 122-CL4. Furthermore, one of thesecond-upper electric-conduction plate 122-CL3 and the first-lowerelectric-conduction plate 122-CL2 can be removed, and therefore themicro-deflector array 122_D and the micro-compensator array 122_C sharethe left one. FIG. 10B shows the case where the first-lowerelectric-conduction plate 122-CL2 is remained. Moreover, thesecond-lower electric-conduction plate 122-CL4 can be removed, and itsfunction can be performed by the beamlet-limit electric-conduction plate121 which can be placed to touch the second-lower insulator plate122-IL4.

FIG. 11A shows one overall embodiment of the source-conversion unit 120in FIG. 8 , which is basically similar to the one in FIG. 10A except thebeamlet-limit electric-conduction plate 121 is between themicro-deflector array 122_D and the micro-compensator array 122_C. Inthis case, each second-upper through-hole can have a cylindrical shape,but an upside-down funnel shape as shown in FIG. 11A is preferred forreducing scattering of stray electrons. FIG. 11B shows one morecompacted embodiment. The micro-deflector array 122_D, the beamlet-limitelectric-conduction plate 121 and the micro-compensator array 122_C areconnected together. The first-lower electric-conduction plate 122-CL2and the second-upper electric-conduction plate 122-CL3 are removed andthe functions thereof are performed by the beamlet-limitelectric-conduction plate 121.

In summary, the source-conversion unit of a conventional multi-beamapparatus in CROSS REFERENCE performs the image-forming function and theaberration-compensation function simultaneously, and therefore theaberration-compensation function generates undesired aberrations due tobeamlet tilting/shifting. This invention modifies the source-conversionunit to perform the image-forming function and aberration-compensationfunction separately. The image-forming function is carried out after theimage-forming function has deflected each beamlet to be locally on-axiswith respect to one corresponding micro-compensator. Hence, other thanthe desired field curvature aberration and the astigmatism aberration,each micro-compensator will not generate additional aberrations due tobeamlet tilting/shifting. The additional aberrations will increase thesizes of plural probe spots and deteriorate image resolution of theapparatus. Accordingly, in one modified source-conversion unit, theimage-forming means comprises one micro-deflector array with pluralmicro-deflectors and one micro-compensator array with pluralmicro-compensators, and each micro-deflector is aligned with onemicro-compensator. Each micro-deflector and each micro-compensatorrespectively is a multipole lens. To make the modified source-conversionunit easy in manufacturing and electric control, each micro-compensatorcan be formed by two or more sub micro-compensators, and accordingly themicro-compensator array comprises two or more micro-compensator layerseach with plural sub micro-compensators. To keep the modifiedsource-conversion unit safe in electric control and less in interferenceof deflecting and compensating fields, the electrodes of all themicro-deflectors and micro-compensators can be covered byelectric-conduction plates. The invention also proposes a method toreduce the impact of Coulomb Effect as much as possible in onemulti-beam apparatus. A Coulomb-effect-reduction means with pluralCoulomb-effect-reduction openings is placed close to the single electronsource of the apparatus and therefore the electrons not in use can becut off as early as possible.

Although the present invention has been explained in relation to itspreferred embodiment, it is to be understood that other modificationsand variation can be made without departing the spirit and scope of theinvention as hereafter claimed.

What is claimed is:
 1. A source-conversion unit comprising: a firstarray having a lens configured to: deflect an off-axis electron beamletof a plurality of electron beamlets; and compensate a first portion ofaberrations of a probe spot formed on a sample by the deflected off-axiselectron beamlet.
 2. The source-conversion unit of claim 1, furthercomprising a second array configured to compensate a second portion ofaberrations of the probe spot formed by the deflected off-axis electronbeamlet.
 3. The source-conversion unit of claim 1, wherein the deflectedoff-axis electron beamlet travels along a local optical axissubstantially parallel to a primary optical axis.
 4. Thesource-conversion unit of claim 1, wherein the first array is furtherconfigured to convert an electron beam generated by an electron sourceinto the plurality of electron beamlets.
 5. The source-conversion unitof claim 4, wherein the first array is further configured to deflect theoff-axis electron beamlet to form a virtual image of the electronsource.
 6. The source-conversion unit of claim 4, wherein the firstarray is further configured to deflect a plurality of off-axis electronbeamlets to form a plurality of virtual images of the electron source.7. The source-conversion unit of claim 6, wherein each of the pluralityof virtual images is associated with a corresponding deflected off-axiselectron beamlet.
 8. The source-conversion unit of claim 1, wherein thefirst array is further configured to add a first compensating-aberrationto the deflected off-axis electron beamlet to reduce the first portionof aberrations, the first compensating-aberration comprising anastigmatism aberration component and a field-curvature aberrationcomponent.
 9. The source-conversion unit of claim 1, wherein the firstarray comprises a plurality of round lenses, a plurality of quadrupolelenses, a plurality of octupole lenses, or a plurality of pairs ofquadrupole lenses.
 10. The source-conversion unit of claim 9, whereineach of the plurality of quadrupole lenses is oriented at a 45° azimuthangle along a plane substantially perpendicular to a primary opticalaxis.
 11. The source-conversion unit of claim 1, wherein the first arraycomprises: a round-lens configured to generate a round-lens field tocompensate a field-curvature aberration of the first portion ofaberrations of the probe spot; and a quadrupole lens configured togenerate a quadrupole field to compensate an astigmatism aberration ofthe first portion of aberrations of the probe spot.
 12. Thesource-conversion unit of claim 11, wherein the round lens is furtherconfigured to generate a primary round-lens field and the quadrupolelens is further configured to generate a secondary round-lens field, andwherein the field-curvature aberration is compensated by the primary andthe secondary round-lens fields.
 13. The source-conversion unit of claim2, wherein the second array is further configured to add a secondcompensating-aberration to the deflected off-axis electron beamlet toreduce the second portion of aberrations, the secondcompensating-aberration comprising an astigmatism aberration componentand a field-curvature aberration component.
 14. The source-conversionunit of claim 2, wherein the second array comprises a plurality of roundlenses, a plurality of quadrupole lenses, a plurality of octupolelenses, or a plurality of pairs of quadrupole lenses.
 15. Thesource-conversion unit of claim 14, wherein each of the plurality ofquadrupole lenses is oriented at a 45° azimuth angle along a planesubstantially perpendicular to the primary optical axis.
 16. Thesource-conversion unit of claim 2, wherein the second array furthercomprises a plurality of micro-compensator layers, each of the pluralityof micro-compensator layers comprising a lens having an electrode. 17.The source-conversion unit of claim 16, wherein the plurality ofmicro-compensator layers comprises: a first micro-compensator layercomprising a first lens and a second lens; and a secondmicro-compensator layer comprising a third lens and a fourth lens,wherein each of the first, second, third and the fourth lens is aquadrupole lens.
 18. The source-conversion unit of claim 2, wherein thesecond array further comprises: a round-lens configured to generate around-lens field to compensate a field-curvature aberration of thesecond portion of aberrations of the probe spot; and a quadrupole lensconfigured to generate a quadrupole field to compensate an astigmatismaberration of the second portion of aberrations of the probe spot. 19.The source-conversion unit of claim 18, wherein the round lens isfurther configured to generate a primary round-lens field and thequadrupole lens is further configured to generate a secondary round-lensfield, and wherein the field-curvature aberration is compensated by theprimary and the secondary round-lens fields.
 20. The source-conversionunit of claim 1, wherein the first array is further configured todeflect the off-axis electron beamlet such that the off-axis electronbeamlet travels substantially parallel to a primary optical axis.